Chicago Gangster underworld?

In the floor behind the bar at the Green Mill Cocktail Lounge, a century-old jazz club in Uptown, lies a door. Beneath it: a musty labyrinth of gangster and Uptown history.

The World Below -- a series of tunnels branching underground from the Green Mill to the bookstore Shake, Rattle & Read a few doors away -- mixes myth and fable, dusty boilers and blood-splattered urinals (more on this in a moment).

In the mid-1910s, the Green Mill was an exclusive hangout for Essanay Studio executives and early film stars such as Charlie Chaplin and Wallace Beery. In recent decades, jazz musicians such as Clifford Jordan, Branford Marsalis and Harry Connick Jr. have graced its stage. But tales of Jazz Age Chicago, when gangsters such as "Machine Gun" Jack McGurn and boss Alphonse Capone defied Prohibition, are most prominent down below.